Anchored oxide coatings on hard metal cutting tools

ABSTRACT

A cutting tool insert comprises a hard metal substrate having at least two wear-resistant coatings including an exterior ceramic coating and a coating under the ceramic coating being a metal carbonitride having a nitrogen to carbon atomic ratio between 0.7 and 0.95 which causes the metal carbonitride to form projections into the ceramic coating improving adherence and fatigue strength of the ceramic coating.

This application claims benefit of provisional application 60/005,952,filed Oct. 27, 1995.

FIELD OF THE INVENTION

The present invention relates to the field of cutting tools andparticularly to coatings for ceramic coated hard metal cutting toolinserts used for cutting, milling, drilling and other applications suchas boring, trepanning, threading and grooving.

BACKGROUND OF THE INVENTION

Coatings improve the performance of cutting tools, especially ceramic oroxide coatings on carbide or hard metal cutting tools. Ever sincecarbide cutting tool inserts have been ceramic coated with, for example,aluminum oxide (Al₂ O₃), there has been a continuing effort to improvethe adherence of the coating to the substrate. When the first aluminumoxide coating was applied directly to a substrate of the carbide or hardmetal type, the oxygen in the aluminum oxide reacted with the substratewhich reduced the adherence.

It has been known to improve the properties of tool inserts made from asintered hard metal substrate (metallic carbide bonded with a bindermetal) by applying a wear-resistant carbide layer. See UK Patents Nos.1,291,387 and 1,291,388 which disclose methods of applying a carbidecoating with improved adherence; specifically, controlling thecomposition of the gas used for deposition of the carbide so that adecarburized zone was formed in the sintered hard metal at the interfacewith the wear-resistant carbide. The decarburized zone known as an etalayer, however, tends to be hard and brittle resulting in breakage. Ithas also been known to apply a ceramic or oxide wear-resistant coating(usually aluminum oxide) upon the sintered metal substrate. However, asalready explained, the oxide layer directly upon the sintered metal bodymay disrupt the sintered metal morphology and binding ability. A numberof patents have disclosed the use of an intermediate layer of carbides,carbonitrides and/or nitrides. See U.S. Pat. Nos. 4,399,168 and4,619,866. An intermediate titanium carbide (TiC) layer improvedtoughness but still an eta layer existed limiting the application of thecoated tool inserts to finishing cuts. A layer of titanium nitride (TiN)applied before the TiC layer eliminated the eta layer but toughness wasstill less than required. See U.S. Patent No. 4,497,874. Intermediatelayers of titanium carbonitride (TiCN) in place of the TiC intermediatelayer have been proposed. See U.S. Patents Nos. 4,619,866 and 4,399,168.A thin surface oxidized bonding layer comprising a carbide or oxycarbideof at least one of tantalum, niobium and vanadium between the hard metalsubstrate and the outer oxide wear layer has been proposed. See U.S.Pat. No. 4,490,191.

The ceramic coating (Al₂ O₃) does not adhere well enough to the TiC andmany TiCN intermediate coatings when used to enhance the adhesion of thecoating to the cemented carbide substrate. Due to thermal expansiondifferences, there is a tendency to delaminate. With the stress causedby the thermal expansion difference, coatings tend to performinconsistently. These intermediate coatings are mostly characterized bya straight line interface between the intermediate coating and the oxidecoating as shown in FIG. 1. This results in a weak bond. Adhesion may beincreased some by making the substrate rough but the projectionsprovided by the roughening are spaced too far apart to performconsistently.

With the coatings, according to the present invention, increased wearresistance as well as adhesion strength are provided in ceramic coatingson hard metal cutting tools.

SUMMARY OF THE INVENTION

Briefly, according to this invention, there is provided a cutting toolinsert comprising a hard metal substrate having at least twowear-resistant coatings. One of the coatings is a ceramic coating. Anintermediate coating under the ceramic coating is comprised ofcarbonitride having a nitrogen to carbon atomic ratio between about 0.7and about 0.95 whereby the carbonitride coating forms fingersinterlocking the ceramic coating, thus improving the adherence andfatigue strength of the ceramic coating. Preferably, the nitrogen tocarbon atomic ratio in the carbonitride coating lies between about 0.75and 0.95 as determined by X-ray diffraction.

According to one embodiment of this invention, the hard metal cuttingtool insert has two intermediate coatings between the hard metalsubstrate and the aluminum oxide surface coating. The coating adjacentthe substrate is a 1 to 4 micron layer of titanium nitride. The coatingover the titanium nitride layer is a 2 to 4 micron thick titaniumcarbonitride layer and the aluminum oxide coating is a 1 to 10 micronlayer.

According to a preferred embodiment, the hard metal substrate of thecutting tool insert has four coatings as follows: a 2 micron titaniumnitride interior coating, a 3 micron titanium carbonitride intermediatecoating, a 6 micron aluminum oxide intermediate coating, and a 2 micronTi (C,N), i.e., TiC, TiN, TiC_(x) N_(y) exterior coating.

Titanium is not the only suitable metal for use in the carbonitridecoating. The metal may be comprised of, in addition to titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenumand tungsten.

The cutting tool insert substrate, according to this invention,typically comprises 3% to 30% of a binder metal from the iron groupincluding, in addition to iron, nickel and cobalt and mixtures thereofand between 70% and 97% of a carbide selected from the group tungstencarbide, titanium carbide, tantalum carbide, niobium carbide, molybdenumcarbide, zirconium carbide and hafnium carbide. In addition to carbides,the cutting tool insert substrate may also include nitrides.

According to a preferred embodiment, the cutting tool insert substratehas a binder phase enriched surface layer, that is, a surface layerenriched with a higher percentage of cobalt or other binder metal.

Briefly, according to this invention, there is provided a method ofmaking a coated cutting tool insert having a wear-resistant coatingcomprising the steps of depositing a metal carbonitride coating having anitrogen to carbon atomic ratio between about 0.7 and about 0.95 byadjusting the reactants used for chemical vapor deposition of saidcoating and depositing a ceramic coating directly over said carbonitridecoating whereby said carbonitride coating and ceramic coating haveinterlocking microscopic fingers.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and other objects and advantages of this invention willbecome clear from the following detailed description made with referenceto the drawings in which:

FIG. 1 is a photomicrograph of a polished section of a hard metalcutting tool insert having an oxide coating and an intermediate coatingaccording to the prior art; and

FIGS. 2-4 are photomicrographs of polished sections of hard metalcutting tool inserts, according to this invention, having anintermediate coating and an oxide coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to this invention, hard metal cutting tools with a ceramic oroxide wear-resistant coating have a novel reinforcing intermediatecoating. The hard metal substrate has a thin metal nitride coatingoverlaid with a titanium carbonitride coating. The wear-resistantceramic coating overlays the metal carbonitride coating. The metalcarbonitride intermediate layer is provided with a nitrogen to carbonatomic ratio that results in superior adherence of the oxide coating dueto the development of interlocking fingers between the oxide coating andthe metal carbonitride coating.

A test was devised to quantitatively evaluate the performance of ceramiccoated hard metal cutting tool inserts. The test is performed on aturning machine. The stock is a cylindrical bar having a diametergreater than about 4 inches. The bar has four axial slots 3/4 inch wideand 11/2 inches deep extending the length of the bar. The bar is mediumcarbon steel AISI-SAE 1045 having a hardness of 25-30 HRC. The tools tobe tested were used to reduce the diameter of the stock as follows.

    ______________________________________    Feed Rate                Speed    (inches per    Depth of Cut                             (surface feet per    revolution or IPR)                   (inches)  minute or SFM)    ______________________________________    .020           .050      500    ______________________________________

It should be apparent that four times per revolution of the stock, thecutting tool insert impacts the edge of a slot. The cutting tool insertis run until it breaks through the coating or another failure isobserved. Failures were observed in the following described test andwere of the fretting type which is a precursor to the greater wear andcutting failure type.

In the following examples, the nitrogen to carbon atomic ratio in thetitanium carbonitride intermediate layer or coating was determined byuse of X-ray diffraction to first detect the lattice spacing of thecarbonitride layer and then to calculate the atomic ratio of nitrogen tocarbon or the atomic percentage of nitrogen based upon nitrogen andcarbon. The lattice spacing of titanium carbide is known to be 1.53Angstroms and the lattice spacing for titanium nitride is known to be1.5 Angstroms. The range or difference is 0.03 Angstroms. Thus, atitanium carbonitride layer found to have a lattice spacing of 1.5073Angstroms is 0.0227 Angstroms between the spacing for titanium nitrideand titanium carbide. Hence, the atomic ratio of nitrogen to carbon is0.0227 divided by 0.03 or 75.7% nitrogen based on total carbon andnitrogen in the carbonitride layer.

EXAMPLE I Comparative Example

A tungsten carbide based substrate (94% tungsten carbide, 6% cobalt) ofK20 material (K20 is a designation of the type of hard cutting materialfor machining as set forth in ISO Standard IS0513:1991(E) classifiedaccording to the materials and working conditions for which the hardmetal cutting material can appropriately be used) was coated accordingto well-known procedures in a Bernex Programmat 250 coating furnace. Thecoating process known as chemical vapor deposition (CVD) was used wheregasses and liquids (converted to gas) are passed over substrates to becoated at 800° to 1,1000° C. and reduced pressures from 50 to 900 mBAR.The reactions used to coat the hard metal substrate were as follows:

    CVD of TiN-uses H.sub.2 +N.sub.2 +Titanium Tetrachloride (TiCl.sub.4)

    CVD of TiCN-uses H.sub.2 +N.sub.2 +TiCl.sub.4 +Acetonitrile (CH.sub.3 CN) or CH.sub.4

    CVD of Al.sub.2 O.sub.3 -uses H.sub.2 +HCl+Aluminum Chloride (AlCl.sub.3)+CO.sub.2 +H.sub.2 S

The essential coating periods and atmospheres used to apply the titaniumnitride layer, the titanium carbonitride layer and the oxide layer areset forth in the following Tables I, II and III. The gas reactants, theproduct of the AlCl₃ reactor and the liquid reactions are introduced tothe furnace.

                  TABLE I    ______________________________________            Run Time    Millibar Reactor                                    ° C.    Coating Minutes     Pressure    Reactor Temp.    ______________________________________    TiN      60         160          920    TiCN    420          60          870    Al.sub.2 O.sub.3            270          60         1005    ______________________________________

                  TABLE II    ______________________________________           Gas Reactants           Liter/Minute    Coating  H.sub.2                   N.sub.2  CO.sub.2                                 CH.sub.4                                         HCl  H.sub.2 S    ______________________________________    TiN      14    9    TiCN     14    8    Al.sub.2 O.sub.3             11             0.6          .20  0.050    ______________________________________

                  TABLE III    ______________________________________    AlCl.sub.3 Gas Generator                         Liquid Reactants    l/min                ml/min    Coating           H.sub.2 HCl       CH.sub.3 CN Liquid                                      TiCl.sub.4 Liquid    ______________________________________    TiN                               2.1    TiCN                     125      2.4    Al.sub.2 O.sub.3           1.9     0.8    ______________________________________

X-ray analysis of the titanium carbonitride layer demonstrated a latticespacing of 1.516 Angstroms which, based on the analysis explained above,represents a nitrogen to carbon atomic ratio of 14:30 or a nitrogencontent of 46.7% based on the total carbon and nitrogen in thecarbonitride layer. The coated tool according to this example wassubmitted to the above-described machining test. After only 14.5seconds, fretting was displayed.

FIG. 1 is a photomicrograph of a polished section showing the layers orcoatings over the substrate. Notice that the interface between thetitanium carbonitride and oxide layer is almost a straight line, thatis, there are no interlocking fingers.

EXAMPLE II

A coating, according to this invention, was prepared on a tungstencarbide based substrate in the coating furnace above described with thecoating periods and atmospheres as described in Tables IV, V and VI.

                  TABLE IV    ______________________________________            Run Time    Millibar Reactor                                    ° C.    Coating Minutes     Pressure    Reactor Temp.    ______________________________________    TiN      60         160          920    TiCN    240          80         1005    Al.sub.2 O.sub.3            540          60         1005    ______________________________________

                  TABLE V    ______________________________________           Gas Reactants           Liter/Minute    Coating  H.sub.2                    N.sub.2  CO.sub.2                                  CH.sub.4                                         HCl  H.sub.2 S    ______________________________________    TiN      14     9    TiCN     11.3   8             0.6    Al.sub.2 O.sub.3             11              0.6         0.2  .050    ______________________________________

                  TABLE VI    ______________________________________    AlCl.sub.3 Gas Generator                         Liquid Reactants    l/min                ml/min    Coating           H.sub.2 HCl       CH.sub.3 CN Liquid                                      TiCl.sub.4 Liquid    ______________________________________    TiN                               2.1    TiCN                              0.9    Al.sub.2 O.sub.3           1.9     0.8    ______________________________________

Tables IV, V and VI, in addition to showing the run times, reactionpressures and temperatures, show the rate of gas reactants, aluminumchloride generator reactants and the liquid reactants. The gas reactantsintroduced into the aluminum chloride generator flow over aluminum metalchips producing a quantity of aluminum chloride which is passed into thecoating furnace.

X-ray analysis of the titanium carbonitride layer demonstrated a latticespacing of 1.5073 which, based on the analysis explained above,represents a nitrogen to carbon ratio of 23:30 or a nitrogen content of75.7% based upon the total carbon and nitrogen in the carbonitridelayer.

The coated tool insert was submitted to the above-described machiningtest. The cutting test showed no fretting at 180 seconds. FIG. 2 is aphotomicrograph of a polished section showing the layers of coating overthe substrate. The photomicrograph illustrates fingers or anchors of thetitanium carbonitride layer penetrating the oxide layer and anchoring itin place.

EXAMPLE III

Example III was prepared the same as Example II except the nitrogen waslower in the coating furnace during the deposition of the carbonitridelayer. The lattice spacing in the titanium carbonitride layer was foundto be 1.509 which represents a nitrogen to carbon atomic ratio of 21:30or a nitrogen content of 70%.

In the machining test, fretting was displayed only after a 5 inch cutlength (estimated 40 to 50 seconds). The microstructure of Example IIshown in FIG. 3 anchors between the oxide and the titanium carbonitridelayers are displayed but are very minor.

EXAMPLE IV

Example IV was prepared the same as Example II except with increasednitrogen flow. The lattice spacing of the titanium carbonitride layerwas 1.503 Angstroms which represents a nitrogen to carbon atomic ratioof 27:30 or 90% nitrogen. In the machining test, the tool insertdisplayed no fretting after 120 seconds. The microstructure of ExampleIV is shown in FIG. 4 and illustrates prominent fingers or anchorsextending between the carbonitride layer and the oxide layer.

EXAMPLE V

In the following example, tool inserts coated according to thisinvention were machine tested with the following cutting conditions. Thestock was 3,000 gray cast iron 200 BHN. The tools tested were used toreduce the diameter of the stock as follows.

    ______________________________________    Feed Rate                Speed    (inches per    Depth of Cut                             (surface feet per    revolution or IPR)                   (inches)  minute or SFM)    ______________________________________    .022           .100      950    ______________________________________

Two steel inserts, according to this invention, ran 108 pieces per edge.By comparison, a C-5 alumina coated tool insert ran 50 pieces per edge.The tool inserts, according to this invention, were a 100% improvement.

EXAMPLE VI

In the following example, the stock for the machining test was ARMAsteel 250 BHN. The machining conditions were as follows.

    ______________________________________    Feed Rate                Speed    (inches per    Depth of Cut                             (surface feet per    revolution or IPR)                   (inches)  minute or SFM)    ______________________________________    .010           .100      1,200    ______________________________________

Using the tool inserts, according to this invention, 170 pieces per edgewere run. By comparison, with C-5 alumina coated tool inserts, 85 piecesper edge were run. The tool inserts, according to this invention, were a100% improvement.

Having thus described our invention with the detail and particularityrequired by the Patent Laws, what is desired protected by Letters Patentis set forth in the following claims.

What is claimed is:
 1. A cutting tool insert comprising a hard metalsubstrate having at least two wear-resistant coatings including anexterior ceramic coating and a coating under the ceramic coating being ametal carbonitride having a nitrogen to carbon atomic ratio between 0.7and 0.95 which causes the metal carbonitride to form projections intothe ceramic coating whereby improving adherence and fatigue strength ofthe ceramic coating.
 2. The cutting tool insert as set forth in claim 1,wherein the metal carbonitride has a nitrogen content of between 70% and90% based upon the total nitrogen and carbon content of the metalcarbonitride layer.
 3. The cutting tool insert as set forth in claim 1,wherein the metal carbonitride has a nitrogen to carbon atomic ratiobetween 0.75 and 0.95 as determined by X-ray diffraction.
 4. A cuttingtool insert as set forth in claim 1, having a coating of titaniumnitride 1 to 4 microns thick, a titanium carbonitride coating 2 to 4microns thick, and an aluminum oxide coating of 1 to 10 microns thick.5. A cutting tool insert according to claim 3, having a titanium nitridecoating 2 microns thick, a titanium carbonitride coating 3 microns thickand an aluminum oxide coating 6 microns thick with an overcoating of Ti(C,N) 2 microns thick.
 6. A cutting tool insert as set forth in claim 1,wherein the metal in the metal carbonitride coating is selected from oneof the groups IVB, VB and VIB in the periodic table of elements.
 7. Acutting insert as set forth in claim 6, and including a substratecomposed of 3% to 30% binder metal from the iron group and between 70%and about 97% carbide selected from the group tungsten carbide, titaniumcarbide, tantalum carbide, niobium carbide, molybdenum carbide,zirconium carbide and hafnium carbide.
 8. A cutting tool insert as setforth in claim 7, wherein in the substrate nitrides replace a portion ofthe carbides.
 9. A cutting tool insert as set forth in claim 6, whereinthe surface layer of the substrate is enriched with the binder metal.10. A method of making a cutting tool insert comprising the steps ofapplying a titanium nitride coating, a metal carbonitride coating and aceramic coating, all by chemical vapor deposition, wherein the reactantsduring the chemical vapor deposition of the carbonitride layer arecontrolled to provide a nitrogen to carbon atomic ratio between 0.75 and0.95 and wherein a ceramic coating is deposited thereover such that thecarbonitride layer has fingers which extend into the ceramic coatingincreasing coating adhesion.